Radiation entrapping, multi-reflection raman sample cell employing a single concave mirror

ABSTRACT

Apparatus for passing light repeatedly through a sample in a sample space comprises two longitudinally separated reflectors one of which is concave and the other of which is planar, the reflectors located to repeatedly reflect a light beam therebetween for passage repeatedly through the sample space.

I United States Patent Siegler, Jr.

[54] RADIATION ENTRAPPING, MULTI- REFLECTION RAMAN SAMPLE CELL EMPLOYINGA SINGLE CONCAVE MIRROR [72] Inventor: Edouard Horace Siegler, Jr.,Claremont, Calif.

[73] Assignee: Cary Instruments, Monrovia, Calif. [22] Filed: Feb. 19,1970 [21] Appl. No.: 12,628

[52] US. Cl ..356/244 51 InLCl ..G01n21/24 '[s8 FieldofSearch..331/94.5;356/75,103,102, 356/244, 246; 350/294 [56] References CitedUNITED STATES PATENTS 3,414,354 12/1968 Siegler 356/75 1 i 16 T 15' j I25; i 1

[451 .HP99-s51 97? 3,428,914 2/1969 Bell ..33l/94.5

OTHER PUBLICATIONS Primary Examiner-William L. Sikes AssistantExaminer-Orville B. Chew, II

Attorney-White & Haefliger [57] ABSTRACT Apparatus for passing lightrepeatedly through a sample in a sample space comprises: twolongitudinally separated reflectors one of which is concave and theother of which is planar, the reflectors located to repeatedly reflect alight beam therebetween for passage repeatedly through the sample space.

15 Claims, 4 Drawing Figures RECORD ER v .1 RADIATION ENTRAPPING, MULTIREFLECTION RAMAN SAMPLE CELL EMPLOYING A SINGLE CONCAVE MIRRORBACKGROUND OF THE INVENTION This invention relates generally tospectroscopy and more particularly concerns enhancement of sampleillumination in spectroscopic systems, the invention having'a highdegree of utility Raman spectrophotomel0 radiation as well as efficientdetection as by means of a 'photodetector. The exciting radiation isthen caused to pass many times through the small sample, and in suchmanner as to trap the exciting radiation so that it cannot leave thesample or the optical system which produces the trapping effect.

The Chupp system'can be relatively expensive to produce in large parttothe need for two concave reflectors of extremely high reflectance, bothrequired to operate in air. Additionally, the Chupp system is subject tothe necessity for particularly accurate and expensive mounting andalignment means for each mirror; andthe number'of passes of the beamthrough interfaces of-high refractive index undesirably reduces thetransmission efficiency of the system.

SUMMARY OF THE INVENTION for the monochromator, but eliminating theglass-toair-and-retum pass at the far side of the sample cell andsubstituting reflection ata dielectric mirror for that at theunprotected far-side mirror. Accordingly, the cost of mirrors issubstantially reduced, with no significant performance loss.

Basically, then, the invention is embodied in the combination thatcomprises two longitudinally separated reflectors one of which isconcave (for example spherical or ellipsoidal) and the other of which isplanar, the reflectors located to reflect a beam of light repeatedlybetween them for repeated passage through the sample space. In thecaseof the spherical reflector, its center of curvature lies in the samplespace between the reflectors and close to the planar reflector, and inthe case of the ellipsoidal reflector it has an actual focus in thesample space between the reflectors and a virtual focus at the far sideof the planar mirror, the latter being half-way between the two foci.

In addition, the sample space may be defined by a sample cellhavingspaced windows through which the beam repeatedly passes, theplanar mirror being a DRAWING DESCRIPTION FIG. 1 is partly an elevation,partly an isometric, and partly a schematic drawing of an embodimentillustrating the principles of the invention;

FIG. 2 is a view showing a sample holder with dielectric mirrorattached;

FIG. 3 is an elevational view similar to part of FIG. 1, butillustrating use of an ellipsoidal mirror; and

FIG. 4 is an elevational view showing a modified form of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring first to FIG. 1, aconcave reflector 10 and a planar reflector 11 are shown as havinglongitudinal separation along a line 14 passing through two points 12and 13. Reflector 10 may comprise a sphericalmirror defining conjugateimage point 12 and center'of curvature point 13.

A sample fluid such as liquid or gas is to be received in whole orinpart in the space 15 between the reflector 11 and point 12 in order thatlight, especially laser light, trapped to pass'repeatedly through-point12 and space 15 may pass repeatedly through the sample to result in highintensity level Raman-light emission from the sample in the lateraldirection 16. The broken line 17 schematically indicates one dispositionof a sample container characterized in that the bulkof the sampleextends within and throughout the major length of space 15 forenhancement of emitted Raman light intensity. Note in this regard thatthe center of curvature. 13 of spherical reflector 10 lies between. thereflectors and relatively close to the planar reflector 1 1.

- Also contemplated is the provision of means to direct such light in abeam for passage through space 15 and repeated reflection by themirrors. As one example, a laser is indicated at 18 directing a coherentlight beam 19 through an opticalsystem 20 (as for example a lens orlenses) operating to focus the beam for reflection by the mirror andpassage at 22 through conjugate image point 12. The direction of thebeam 19 is off-axis" at an acute angle a relative to linear axis 14.Beam 22 is reflected at 23 on mirror 10 to pass at 24 back to mirror 11for reflection at 25 and return at 26 through point 12. As is evident,repeated reflections occur with the light beam converging toward line14, to give maximum illumination of zone 15 by entrapment. of theexciting light. While some slight magnification will occur upon eachreflection, if the spacing of point 12 from mirror 11 is minimized, orsmall, relative to the longitudinal spacing of mirror surfaces 10 and.11, the magnification effects are minimized and the reflected light canbe caused to pass between 10 and times through point 12 and space 15,before the successive magnifications, causing the beam diameter toexceed the width of the slit image, and reflection losses at the mirrorsreduce the usable residual light flux to a small fraction of itsoriginal level. This number of passes is definitive of.a very efficientlight collecting cell.

Means to collect light (as for example Raman light) transmitted at 16may' for example include a monochromator 123 to which light is directedby optical system 124. The monochromator operates to disperse theradiation from the illuminated sample, and in so doing to reduce theintensity of undesired background radiation compared with the intensityof the spectral lines being detected, as referred to in U. S. Pat. No.2,940,355 to H. H. Cary. In this regard, photodetector 125 receivesdispersed radiation at 126 from the monochromator; an amplifier 27 isconnected to the output of the photodetector; and a recorder 28 isconnected to the amplifier output. The photodetector serves to generatean electrical signal corresponding to the intensity of at least aportion (as for example the Raman portion) of the light emitted at 16from the sample.

For highest 'efficiency, the spacing of the conjugate image point 12from mirror 1 1 may be such, in relation to the length of themonochromator entrance slit 29,

that the reduced slit image found at zone has opthe gap between point 12and the mirror, and conversely, images the gap between point 12 and themirror at the slit. The monochromator exit slit is designated at 30.

The separation of the two mirrors is in principal onehalf the separationas calculated for a system using two concave mirrors, as described inVernon L. Chupp application Ser.. No. 832,102 for U. 8. Letters Patent,filed June ll, 1969. Also, the spacing of point 12 from the mirror 11 isone-half the spacing between a pair of conjugate image points asdescribed in that Chupp application, for either liquid or gas samples.Compensation for the differences in refractive indices of differentsample liquids in the sample cell, and for the refractive index of thematerial of the cell windows, may be made by moving either of themirrors and/or the sample cell along axis 14, as described in the Chuppapplication. Devices to so move the mirrors are schematically indicatedat 32 and 33.

One usable sample cell is illustrated at 17a in FIG. 2, the lanar irror11a being a dielectric mirror carried on the outwardly facing side 34 ofthe cell wall 35. Walls 35 and 36 at opposite sides of the sample space15a define cell windows through which the beam repeatedly passes, andthe beam-passing surfaces of those windows are parallel to the planarmirror 11a. Such surfaces are typically glassy, and may for exampleconsist of fused silica. A device to adjustably displace the sample cellalong the direction of axis 14a is indicated schematically at 37.

FIG. 3 illustrates an apparatus similar to that described in FIGS. 1 and2, and including a sample cell 17b, a planar mirror 11b carried on cellwall 35b, a sample space 15b in the cell; however, the concave mirror10b is in this case ellipsoidal.

The image point 12b is now at a focus of the ellipsoidal mirror, theother focus being virtual and located at point 12c within a virtualimage 17c of the cell 17b.

guished from actual foci, the former being shifted from the latter alongaxis 14 (in the downward direction'as drawn, assuming the concave mirrorto be held fixed), where refractive indices of the sample cell andsample liquid are taken into consideration.

Finally, FIG. 4 illustrates a special case of the invention a modifiedsystem useful only for gaseous specimens, and incorporating a concavemirror 40, a planar mirror 41 carried on the wall 42 of cell 43, and thecell window 44 furthest from mirror 41 being of special character.Window inner surface 46 is parallel to window outer surface 45, bothbeing-angled relative to the beam 47 in such relation that thereflection loss resulting from incidence of the bean on either surfaceis essentially zero. To obtain this condition, planar window surfaces 46and 45 are made to lie with their normal 56 at the well-known Brewsterangle ,8 relative to beam 47. Note that the beam emanates from laser 48,passes centrally above the mirror (that is, out of the plane of thedrawing, angled toward that plane), and then is directed as shown to bereflected back and forth between the mirrors 40 and 41 and throughsample space 50.

FIG. 4 is a view taken at right angles to the views of the other threefigures, so the converging fan of 'ray paths between the mirrors is notreadily shown but is present, along with typically, the collectionoptics, monochromator and electronics discussed earlier.

The center of curvature or focus of mirror 40 is located exactly asalready described for the general case of the invention, the opticalaxis of the system merely being bent at surfaces and 46 in accordance Iwith refraction of the laser beam at the Brewster window.

Within the cell, the beam paths as at 51 and 52 are parallel (inprojection) to the path 47 outside the cell. When a ray 47 is consideredto be a nearly axial ray, representative of rays occurring aftersubstantial 53. Suitable selection of entrance angle a (FIG. 1) andminimization of window 44 thickness adequately minimizes beam spread 53within the cell.

Operation of the cell in highly efficient Brewsterangle mode, free ofanti-reflection coatings within and without, depends on parallelism ofray 47 with rays 51 and 52 in the projection of FIG. 4, and this in turnrequires nonvariation of refractive index of specimens in space 50, fromone specimen to another. Since liquid specimens have considerable rangeof refractive index, this special-case cell appears to have practicalutility only for gaseous samples, whose refractive index generallyequals (to adequate approximation for this purpose) that of the airoutside the cell.

Reflector 41 is advantageously, placed within the cell to avoidreflection loss at upper surface 54 of wall 42, and the cell structure43 is advantageously provided with one or more ports 49 for admissionand removal of gaseous specimens.

a. two longitudinally separated reflectors one of which is sphericallyconcave and the other of which is planar, the reflectors located toreflect repeatedly a light beam therebetween for repeated passagethrough .the sample space in a substantially planar reflection array,the spherically concave reflector defining a center of curvature and twocoincident conjugate image points, i

. said center of curvature of the spherically concave reflector lyingbetween the reflectors and close to but spaced from the planarreflector,

. and a sample holder defining said sample space between said center ofcurvature and said coincident conjugate image points, the spacing of thereflectors characterized in that successive reflections of the beam passthrough at least one wall of the holder as well as through the samplespace and converge toward a straight line extending through said centerof curvature and said coincident conjugate image points.

2. In apparatus for passing light repeatedly through a sample and samplespace, the combination comprising a. two longitudinally separatedreflectors one of which is ellipsoidal and the other of which is planar,the reflectors located to reflect repeatedly a light beam therebetweenfor repeated passage through the sample space in a substantially planarreflection-array,

. the ellipsoidal reflector having two coincident foci between thereflectors,

. and a sample holder defining said sample space between said coincidentfoci and said planar reflector the spacing of the reflectorscharacterized in that successive reflections of the beam pass through'atleast one wallof the sample holder as well as through the sample spaceand converge said toward a straight line normal to the planar reflectorand passing through said coincident foci, the foci located within saidspace.

, 3. The apparatus of claim 2 wherein the ellipsoidal reflector also hasanother focus, the two foci being effective foci, and the planarreflector located half the distance therebetween.

4-. In apparatus for passing light repeatedly through a sample in asample space, the combination comprising a. two longitudinally separatedmirrors one of which is concave and the other of which is planar, themirrors located to repeatedly reflect a light beam therebetween forpassage repeatedly through said space, the concave reflector definingtwo coincident conjugate image points,

b. a sample cell defining said space through which the beam passesduring its repeated reflection between the mirrors, said space lyingbetween the coincident conjugate image points and said planar reflector,

. the concave mirror being spaced from the cell and the planar mirrorbeing adjacent a surface of the cell, and

. means to direct the beam for passage through said space and forreflection by said mirrors in a substantially planar reflection array,the spacing of the 0 characterized in at successive re ec"o s oi the ampass througli at least one w 0 die cell as well as through the samplespace and converge toward a line normal to said planar mirror andpassing through the coincident conjugate image points, the concavemirror defining a conjugate image point within the sample space.

5. The combination of claim 4 wherein said planar mirror comprises adielectric mirror carried on a surface of the cell.

6. The combination of claim 4 wherein the cell has spaced windowsthrough which the beam repeatedly passes and at opposite sides of saidsample space, the beam-passing surfaces of each window being planar andparallel to said planar mirror.

7. The combination of claim 4 wherein the windows consist of fusedsilica.

8. The combination of claim 4 wherein theconcave mirror is ellipsoidaland said image point is a focus thereof.

9. The combination-of claim 4 wherein the concave mirror is spherical.

10. The combination of claim 4 wherein said beam consists of laserlight, and including means for collecting Rarnan light transmitted froma sample in the sample space.

11. The combination of claim 4 wherein said planar mirror is outside thecell.

12. The combination of claim 4 wherein said planar mirror is inside thecell.

13. The combination of claim 4, said cell having a window through whichthe beam repeatedly passes, said window forming a portion of said cellsubstantially opposite said planar mirror, and said window havingmutually plane-parallel surfaces, both angled relative to the beam insuch relation that the reflection loss resulting from incidence of thebeam on said surfaces is essentially zero.

14. The combination of claim 13 wherein said window is glassy and freeof anti-reflection coatings.

15. The combination of claim 13 wherein the normal to said mutuallyplane-parallel surfaces is at Brewsters angle to said beam.

1. In apparatus for passing light repeatedly through a sample and samplespace, the combination comprising a. two longitudinally separatedreflectors one of which is spherically concave and the other of which isplanar, the reflectors located to reflect repeatedly a light beamtherebetween for repeated passage through the sample space in asubstantially planar reflection array, the spherically concave reflectordefining a center of curvature and two coincident conjugate imagepoints, b. said center of curvature of the spherically concave reflectorlying between the reflectors and close to but spaced from the planarreflector, c. and a sample holder defining said sample space betweensaid center of curvature and said coincident conjugate image points, thespacing of the reflectors characterized in that successive reflectionsof the beam pass through at least one wall of the holder as well asthrough the sample space and converge toward a straight line extendingthrough said center of curvature and said coincident conjugate imagepoints.
 2. In apparatus for passing light repeatedly through a sampleand sample space, the combination comprising a. two longitudinallyseparated reflectors one of which is ellipsoidal and the other of whichis planar, the reflectors located to reflect repeatedly a light beamtherebetween for repeated passage through the sample space in asubstantially planar reflection array, b. the ellipsoidal reflectorhaving two coincident foci between the reflectors, c. and a sampleholder defining said sample spaCe between said coincident foci and saidplanar reflector the spacing of the reflectors characterized in thatsuccessive reflections of the beam pass through at least one wall of thesample holder as well as through the sample space and converge saidtoward a straight line normal to the planar reflector and passingthrough said coincident foci, the foci located within said space.
 3. Theapparatus of claim 2 wherein the ellipsoidal reflector also has anotherfocus, the two foci being effective foci, and the planar reflectorlocated half the distance therebetween.
 4. In apparatus for passinglight repeatedly through a sample in a sample space, the combinationcomprising a. two longitudinally separated mirrors one of which isconcave and the other of which is planar, the mirrors located torepeatedly reflect a light beam therebetween for passage repeatedlythrough said space, the concave reflector defining two coincidentconjugate image points, b. a sample cell defining said space throughwhich the beam passes during its repeated reflection between themirrors, said space lying between the coincident conjugate image pointsand said planar reflector, c. the concave mirror being spaced from thecell and the planar mirror being adjacent a surface of the cell, and d.means to direct the beam for passage through said space and forreflection by said mirrors in a substantially planar reflection array,the spacing of the mirrors characterized in that successive reflectionsof the beam pass through at least one wall of the cell as well asthrough the sample space and converge toward a line normal to saidplanar mirror and passing through the coincident conjugate image points,the concave mirror defining a conjugate image point within the samplespace.
 5. The combination of claim 4 wherein said planar mirrorcomprises a dielectric mirror carried on a surface of the cell.
 6. Thecombination of claim 4 wherein the cell has spaced windows through whichthe beam repeatedly passes and at opposite sides of said sample space,the beam-passing surfaces of each window being planar and parallel tosaid planar mirror.
 7. The combination of claim 4 wherein the windowsconsist of fused silica.
 8. The combination of claim 4 wherein theconcave mirror is ellipsoidal and said image point is a focus thereof.9. The combination of claim 4 wherein the concave mirror is spherical.10. The combination of claim 4 wherein said beam consists of laserlight, and including means for collecting Raman light transmitted from asample in the sample space.
 11. The combination of claim 4 wherein saidplanar mirror is outside the cell.
 12. The combination of claim 4wherein said planar mirror is inside the cell.
 13. The combination ofclaim 4, said cell having a window through which the beam repeatedlypasses, said window forming a portion of said cell substantiallyopposite said planar mirror, and said window having mutuallyplane-parallel surfaces, both angled relative to the beam in suchrelation that the reflection loss resulting from incidence of the beamon said surfaces is essentially zero.
 14. The combination of claim 13wherein said window is glassy and free of anti-reflection coatings. 15.The combination of claim 13 wherein the normal to said mutuallyplane-parallel surfaces is at Brewster''s angle to said beam.